When I had my last rear tyres fitted (on the drive), the fitter was just about to use new weights to balance them, which would have been very noticeable on black wheels. I asked him about this, and he gave me a handful to paint before he fitted them.
The problem is that they were very glossy, but a quick flatting down gave me a nice enough surface to wallop a couple of coats of Halfords finest satin black on.
Not the best of prep, but they still look better than shiny metal.